Well, developer Eric Chee may have you covered here. At least, part of the way there.
You’ll need a Mac to route messages from your iCloud account over to your phone and you’ll also need to jump through a few more hoops like setting up a couple of projects in the IntelliJ IDE and changing values around.
And even after you’ve successfully executed all the string changes you need, you’ll still only have a very basic iMessage client on Android. You can receive group messages, but you can’t send into them. You could also receive and send media, too, but you can’t in this version. And hey, if Apple changes things around in its own code, well, good luck working around that.
Yep, this is all a work-in-progress and it’s at this point that Chee is looking for some help from the community.
The app and its dependent components are at the GitHub link below, so if your iPhone friends still bother you about your green-ness, try and see if you can hash out Pie Message to be the iMessage client many Android users want it to be.